1. Field of the Invention
This invention relates generally to electrical connections to semiconductor devices. More particularly, the invention pertains to methods and apparatus for making nonpermanent and permanent low-resistance interconnections between a semiconductor device (die) and a substrate.
2. State of the Art
As the densities of input/output (I/O) wire bond pads increase on semiconductor devices, testing of the devices becomes more difficult. The function of any interconnect system, whether a probe card, test socket, or bum-in socket, is to provide a reliable interconnect between the integrated circuit tester and the individual semiconductor device. The reliable bum-in and testing of bare dice is required to provide known-good-die (KGD) for incorporation into multi-chip assemblies, for example. The KGD testing of dice and wafers is dependent upon uniformly achieving consistent electric connections between the test apparatus and the semiconductor device substrate.
Prior art contact members for testing dice are generally of four forms. In one form, the contact members simply abut the bond pads or leads and the two are pressed together to make the desired electrical contact. Examples of this type of interconnection are described in U.S. Pat. Nos. 5,406,210 of Pedder, 5,572,140 of Lim et al., 5,469,072 of Williams et al. and 5,451,165 of Cearley-Cabbiness et al.
A problem with such connections is that bond pads are typically covered with a layer of metal oxides or silicon dioxide which insulates the pads and makes simple contact ineffective as a reliable electrical connection. In some cases, differential thermal expansion of the contact member may cause lateral movement which tears the bond pad.
In a second configuration, contact members may be formed to make a xe2x80x9cwiping actionxe2x80x9d contact with the bond pads. Examples of such are various sockets, pins, plugs, etc. Again, as is well known in the industry, pre-existent oxides and subsequently formed oxides on the metal surfaces result in defective electrical contact.
In a third form of making temporary contact between a test device and a die, the contact members are nonpermanently bonded to the pads on the dice by a solder or other conductive bonding material. Illustrative of this configuration is the disclosure of U.S. Pat. No. 5,517,752 of Sakata et al. Removal of the solder (by remelting) is required to disconnect the contact members from the dice after testing is completed.
The use of solder reflow technology for temporary bonds has many disadvantages including the following. First, surface preparation with highly corrosive and environmentally hazardous fluxes is required. Second, solder bonds are occasionally defective, requiring testing of each bond and reworking if necessary. Third, solder reflow requires several additional processing steps and apparatus, adding to the manufacturing expense. Fourth, the temperatures required for reworking as well as disconnect melting place additional stresses on the device.
A fourth form of contact member is configured to puncture a bond pad, passing through an oxide layer into the underlying metal for low-resistance electrical contact. An example of this configuration is shown in U.S. Pat. No. 5,506,514 of Difrancesco, incorporated by reference herein.
One preferred form of a puncturing contact member is described in U.S. Pat. Nos. 5,326,428 of Farnworth et al., 5,478,779 of Akram, and 5,483,741 of Akram et al., all of which are incorporated by reference herein. In this configuration, the interconnect has a non-conductive or semiconductive substrate upon which raised contact members include sharp projections for puncturing the metal oxide coating on the bond pads and retaining non-permanent, low-resistance electrical continuity with the underlying metal. A compressive force is maintained during the time the die or dice are undergoing testing. The sharp projections may be formed to limit the penetration distance.
Generally, the metal oxide layer overlying the metal is much harder than the metal. Thus, the force required to penetrate and pass through the oxide layer is considerably greater than the forces required to penetrate the metal.
The compressive force exerted on the interconnect and the semiconductor die may be controlled by (a) controlling the rate of movement toward each other, or (b) controlling the compressive force itself, such as with a spring or other such device. In either case, the initially high resistance requiring a high compressive force to penetrate the oxide layer is suddenly released upon penetration. However, the compressive force may not be reduced to avoid xe2x80x9cover-penetrationxe2x80x9d of the underlying metal. Furthermore, even small differences in the thickness of the oxide layer will result in oxide penetration at different compression levels. The required compressive force to achieve oxide penetration of all bond pads will vary from die to die, further exacerbating the problem. Such is particularly a problem where the die has a large number of bond pads thereon and the compressive force required to penetrate any oxide coating on the bond pads is larger than that capable of being transmitted through the head of the transfer apparatus.
As is well known in the art, ultrasonic vibration has been used to join bond pads and leads with thin wires. U.S. Pat. Nos. 5,494,207 and 5,607,096 of Asanasavest and U.S. Pat. No. 4,475,681 of Ingle teach particular ultrasonic wire bonding apparatus and methods. Ultrasonic vibration may be combined with heating as in the xe2x80x9cthermosonicxe2x80x9d wire bonding process. U.S. Pat. No. 3,697,873 of Mazur describes a method for ultrasonically soldering contacts and indicates that xe2x80x9cthe ultrasonic wave energy acts to break up oxides on the surface of the semiconductor body . . . .xe2x80x9d
U.S. Pat. No. 3,938,722 of Kelly et al. discloses an apparatus using ultrasonic energy for forming bonds between beam leads and conductive surfaces such as on a substrate.
The invention comprises an apparatus and method for reducing the compressive force required to achieve the desired initial penetration of a bond pad by a contact member, such as used in a die bum-in and testing carrier. The high initial force required in the prior art to break through the xe2x80x9ccrustxe2x80x9d of hard oxide on the surface of the metal pad is much reduced. Penetration of the hard oxide layer ordinarily results in a sudden xe2x80x9creboundxe2x80x9d of accelerated movement due to reduced resistance, and the contact member may overpenetrate the bond pad. However, with this invention, the rebound is minimized, if not eliminated.
The maximum compressive force required to achieve the final desired penetration of the bond pad is also reduced, while ensuring that all of the bond pads on a bare die are fully penetrated to provide uniformly low-resistivity interconnections.
In the invention, ultrasonic vibration is applied to either one die or both of the bare dice and the corresponding interconnect contact member. The vibratory movement is generated in a direction generally perpendicular to the die surface by a transducer including, e.g., a piezoelectric element. This direction of vibratory movement is known herein as the xe2x80x9caxialxe2x80x9d direction.
The frequency and amplitude of the vibratory forces are controlled such that the interconnect contact member pierces the hard oxide layer on the bond pad very rapidly and at lower applied compression forces. The ultrasonic vibration also ensures that all bond pads are fully penetrated to achieve low-resistivity interconnections.
The bonding system of the invention permits the use of low bonding compressional forces together with generally unidirectional ultrasonic vibrational energy and a frequency-modulated, controlled resonance to produce uniformly reliable simultaneous connection of all bonds on a die without heat or with minimal heating.